The present embodiments relate to a local coil for a magnetic resonance system with a plurality of magnetic resonance antenna elements.
Magnetic resonance tomography is a widely used procedure for obtaining internal images of the body. In this procedure, the body to be investigated is exposed to a relatively high base magnetic field (e.g., 1.5 tesla, 3 tesla or more). A high-frequency excitation signal (e.g., the B1 field) is emitted with an antenna device, where the nuclear spins of certain atoms that are resonantly stimulated by this high-frequency field are tilted by a defined flip angle with respect to the magnetic field lines of the base magnetic field. The high-frequency signal emitted during the nuclear spin relaxation (e.g., the “magnetic resonance signal”) is then collected with suitable antenna arrangements. The raw data thus acquired is then used to reconstruct the image data. Magnetic field gradients defined respectively during the transmission and the reading or reception of the high-frequency signals are superimposed on the base magnetic field for the purpose of spatial encoding.
Magnetic resonance antenna arrangements for receiving magnetic resonance signals may be antenna arrangements that are also used for emission of the B1 field. A “whole-body coil” (e.g., a “whole-body antenna” or a “body coil”) may be installed in the scanner unit, in which the magnetic resonance measuring space (e.g., implemented in the form of a patient tunnel) is located, for transmission of the B1 field. The “whole-body coil” may be configured so that the coil emits a homogeneous B1 field in as large an area as possible inside the magnetic resonance measurement space.
“Local coils” are also used in many investigations. During the investigation, the local coils are positioned relatively close to the surface of the body, directly over an area of interest to be investigated (e.g., a specific organ or part of the body). Because of the close proximity to the areas of interest, the noise level caused by the electrical losses within the object of investigation is reduced, so that the signal-to-noise ratio (SNR) of a local coil may be better than the SNR of a more distantly located antenna. An individual antenna element (e.g., in the form of a single conductor loop with a preamplifier) is, however, only able to generate an effective image within a certain area extending approximately over the diameter of the conductor loop. Therefore, to minimize the measurement time with parallel imaging, most local coils are designed as “multichannel coils” with a plurality of individual magnetic resonance antenna elements (e.g., MR antenna elements; many individual conductor loops arranged alongside or overlapping one another in the form of a matrix, each having a corresponding preamplifier).
In order, for example, also to be able to use parallel imaging facilities such as SENSE and GRAPPA procedures, local coils with more and more channels are being developed. Local coils with up to 32 channels or individual antenna elements are used currently. Local coils with up to 128 channels are in planning or being tested. Such local coils may be mechanically constructed in any way, for example, as relatively flexible, flat antenna arrangements that are placed over, under or on the object of investigation. Additionally or alternatively, the local coils may be mechanically constructed as stable, cylindrical constructions for use as head coils or similar. Local coils may be used not only for receiving magnetic resonance signals, but also, with suitable wiring of the MR antenna elements, for emitting the high-frequency signals for excitation.
A high number of receive channels uses a high number of receivers on the side of the magnetic resonance system's receiving device. The receiving device may, in the following, be the complete unit of the magnetic resonance system with several individual receiving channels, in which the received raw data is amplified and, for example, decoded, separated and finally digitized. The data then exists as raw digital data for the reconstruction of image data.
To enable local coils with a higher number of MR antenna elements to be used even with a lower number of receiving channels in the magnetic resonance system, switch matrices (e.g., receive coil switch (RCCS)) and mode matrices may be used. A switch matrix is a piece of hardware that automatically switches the outputs of the currently active MR antenna elements to specific outputs, to which the individual receiving channels of the receiving device are connected. A mode matrix may be a circuit configuration that connects adjacent receiving channels together into “modes.” This is a combination circuit of phase shifters and hybrids that combine the signals according to magnitude and phase so that N modes are generated from N input signals of N MR antenna elements. This signal combination may, for example, already take place in the local coil if a mode matrix circuit is integrated. The first mode (e.g., primary mode or CP mode) already contains the main image information and provides the maximum SNR at the center of the region of interest (ROI) of the patient's body. The higher modes (e.g., secondary mode and the tertiary mode) increasingly provide SNR in peripheral areas of the body and serve to improve the quality of the image and facilitate the use of parallel imaging techniques such as SENSE and GRAPPA. The sum of all mode signals in total contains the same information as the original signals of the individual MR antenna elements. Such a mode matrix is described, for example, in DE 103 13 004.
Because of technical developments, the individual receive channels of the receiving device may be implemented far more cost-effectively in the future. This will provide that using a mode matrix circuit for the purpose of reducing channels will no longer be worthwhile. Secondly, mode formation also has other advantages, since mode formation enables magnetic resonance images to be generated that have a relatively low sensitivity to the patient's skin surface. This is particularly advantageous for reducing motion artifacts (e.g., in the case of high channel coils with a very large number of relatively small MR antenna elements).
In order to be able to generate modes despite the absence of a mode matrix circuit behind the local coils, a digital mode formation may be useful. In this case, the magnetic resonance signals received by the MR antenna elements after the digitization, for example, may also be linked into modes using suitable software or simple FPGAs or ASICs, or similar.